1. Field of the Invention
This invention relates to a curve motion rolling guide unit in which a slider is formed relatively slidably with respect to a spline shaft, which extend in a longitudinally curved manner, via rolling elements.
2. Description of the Prior Art
A ball spline in which a rolling element-carrying slider is moved linearly along a spline shaft has heretofore been applied to an industrial robot and a transfer machine. The known ball splines include a ball spline provided with retainer members for holding a plurality of balls on a slider, and a ball spline not provided with retainer members.
Conventional ball splines not provided with retainer members include a ball spline shown in FIGS. 9 and 10. This ball spline has a spline shaft 51 provided with a plurality of raceway grooves 53 in predetermined axially extending portions of an outer circumferential surface thereof, and an outer cylinder 50 fitted slidably at its hollow 62 around the spline shaft 51. The outer cylinder 50 comprises a casing 52 provided with raceway grooves 57 in an inner circumferential surface thereof and return passages 58, end caps 54 fixed to both end surfaces of the casing 52 and provided with direction changing passages 59, end seals 55 fixed to end surfaces of the end caps and adapted to seal clearances between the spline shaft 51 and outer cylinder 50, and rolling elements 56 endlessly circulated in the raceway grooves 57, direction changing passages 59 and return passages 58.
The casing 52 is provided with a keyway 60 for setting and positioning an object member of an apparatus therein. The casing 52 is further provided in an inner circumferential surface of the hollow 62 with raceway grooves 57 opposed to those 53 in the spline shaft 51. The end seals 55 are fixed to the casing 52 through the end caps 54 by bolts 63. The raceway grooves 57 formed in the casing 52 are provided at their edge portions with opposed claws 61, the distance between which is set smaller than the diameter of the balls 56, for the purpose of retaining the balls 56 which constitute the rolling elements (refer to, for example, Japanese Patent Publication No. 61046/1991).
The known ball splines provided with retainer members include a ball spline shown in FIG. 11. This ball spline comprises a spline shaft 71 provided with a plurality of raceway grooves 73 in predetermined axially extending portions of an outer circumferential surface thereof, and a slider 70 moving slidingly on the spline shaft 71, the slider 70 comprising an outer cylinder 72 provided on the inner side thereof with raceway grooves 77 opposed to those 73 in the spline shaft 71, and inclined surfaces 79 in which direction changing passages 81 for changing the direction of movement of the balls 76 are formed, retainer members 75 for holding the balls 76 on the inner surface of the outer cylinder 72, and a plurality of balls 76.
The retainer members 75 are formed entirely to a thin-walled seamless cylinder having a larger diameter at the portions thereof which correspond to the balls 76 in nonloaded regions of return passages 78 in the retainer members 75, and a smaller diameter at the portions thereof which correspond to the balls 76 in loaded regions of the raceway grooves 77. The retainer members 75 are provided with elongated bores 80 so that the balls 76 rolling in the loaded regions project slightly therefrom. The retainer members 75 are fixed to the outer cylinder 72 by screwing nut covers 74 to both end portions of the inner circumferential surface of the outer cylinder 72 (refer to, for example, Japanese Utility Model Laid-Open No. 52317/1983 and Japanese Patent Publication No. 3247/1995).
Bearings for a curved sliding motion include a bearing disclosed in Japanese Patent Laid-Open No. 186028/1988. This bearing for a curved sliding motion is adapted to guide curvilinearly an object which requires to be moved from a reference position to another objective position at a slider portion of various kinds of machine tools, industrial robots or part transfer systems. This bearing for a curved sliding motion comprises a sliding base having a cover fixed thereto, and provided with endless raceways for balls each of which comprises a loaded ball groove and a nonloaded ball passage, a track rail having ball rolling grooves opposed to the loaded ball grooves, and balls rolling as they bear a four-directional load between the loaded ball grooves and nonloaded ball passages. The track rail is formed so as to have an arcuate or circular cross section, and the center of an imaginary circle passing the loaded ball grooves in the sliding base and ball rolling grooves in the track rail is aligned with that of an imaginary circle passing the outer circumferential surface of the track rail.
The ball spline shown in FIG. 9 is provided with ball retaining portions on the side edge parts of the raceway grooves 57 in the outer cylinder 50, and it is considerably difficult to form ball retaining portion-carrying raceway grooves 57 in the outer cylinder 50. Unlike the raceway grooves 57 in the casing in a linear motion rolling guide unit, the raceway grooves 57 in the ball spline cannot be formed by using a large-diameter grindstone. Therefore, the inner raceway grooves 57 in the outer cylinder 50 are formed by broaching the outer cylinder, and thereafter quenching the resultant product and finishing the quenched product by polishing. The finishing work is difficult, and many manufacturing mandays are required. Since the inner raceway grooves 57 in the outer cylinder in the ball spline are formed by broaching, the axial length with respect to the inner diameter is limited, so that race grooves of a large length cannot be formed.
In the linear motion rolling guide unit shown in FIG. 11, the raceway grooves 77 in the outer cylinder 72 are provided at both of the axial end portions of the ball loaded regions with escape parts, which comprise inclined grooves, for the purpose of guiding the balls 76 moving in the direction changing passages 81 in the retainer members 75. Therefore, the forming the raceway grooves 77 is difficult, and the manufacturing cost becomes high. In the conventional linear motion rolling guide unit, there are difficulties in the broaching work and highly accurate finishing work for the raceway grooves 77 in the outer cylinder in which the retainer members 75 is provided. It is also troublesome to form mount portions, to which the retainer members 75 are fixed, in the outer cylinder 72, and the cost of manufacturing the outer cylinder increases.